Only six apps out of 40 tested were found to be accurate.

Women are at risk of unwanted pregnancy if they're relying on a smartphone app to monitor their fertility levels in place of more traditional contraception, a new study warns.

The study of 40 fertility awareness apps found that most don't employ "evidence-based methodology".

"Smartphone apps are increasing in popularity because more and more women are interested in using natural or fertility awareness based methods of family planning because they want to feel empowered with greater knowledge of their bodies," said lead author Dr Marguerite Duane, a family physician.

But as the authors write: "The effectiveness of fertility awareness based methods (FABMs) depends on women observing and recording fertility biomarkers and following evidence-based guidelines.

"Apps offer a convenient way to track fertility biomarkers, but only some employ evidence-based FABMs."

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Success using FABMs depends on many factors, including the ability to accurately make and classify daily observations.

But the authors say relying solely on an FABM app may not be sufficient to avoid pregnancy.

For the review, more than 95 apps were identified on iTunes, Google, or Google play.

Of those, 55 were excluded from evaluation because they either had a disclaimer prohibiting use for avoiding pregnancy or did not claim to employ an evidence-based FABM.

The researchers evaluated the remaining 40 apps for accuracy using a rating system based on criteria used by Family Practice Management.

Each app was rated on a five-point scale for 10 clearly defined criteria, which were weighted based on their level of importance for avoiding pregnancy.

"Of those reviewed, 30 apps predict days of fertility for the user and 10 do not," the researchers wrote.

"Only six apps had either a perfect score on accuracy or no false negatives (days of fertility classified as infertile)."

Apps that do not predict fertile days scored high on accuracy only if they required women to receive training in FABM prior to using the app.

"When learning how to track your fertility signs, we recommend that women first receive instruction from a trained educator and then look for an app that scored four or more on mean accuracy and authority in our review," said Dr Duane.